There is known a Ni—Fe alloy having very high magnetic permeability, which is generally called Permalloy. For example, the proportion of direct current components is great in a noise filter for high frequency waves which is used in an A-D converter of a switching power supply of a small-size electronic device, and therefore a Ni—Fe alloy having a high saturation magnetization value and high magnetic permeability exhibits its excellent function. Parts of electronic devices, such as a core for a noise filter, are often produced mainly by molding a mixture of an alloy powder and a resin or by compacting an alloy powder by a powder metallurgical process.
A Ni—Fe alloy powder which is used as a material for parts of various electronic devices has hitherto been produced by the gas atomization method or the mechanical pulverization method depending upon a use theref or. However, a Ni—Fe based alloy powder of submicron particle size which has a homogenous composition and high magnetic permeability has not yet been known.
A Ni—Fe based alloy has high ductility and, therefore, it is impossible to pulverize this alloy powder into one having particles with submicron size. Besides, in the pulverization process plastic strains are introduced and magnetic properties deteriorate. Therefore, it was impossible to utilize the high magnetic permeability which Ni—Fe alloys inherently have. In addition, although this powder has good formability, its productivity is low since high temperatures of 1000° C. or more are required in order to obtain sufficient sintered density. A powder produced by the gas atomization method is inferior in compactibility and is not easy to compact. Further, it is impossible to produce thin films of several micrometers in thickness using these powders, because the particle diameter of these conventional powders is usually as large as dozens of micrometers or more.
In the present invention, there is provided a technique for improving the Permalloy alloy, although its magnetic permeability is high, which has drawback properties in high frequency band because of low electrical resistivity, and for making this alloy usable in the MHz (megahertz) band and in the higher frequency bands. For this purpose, it must be ensured that thin films of about 5 μm or less in thickness can be produced. Such thin films cannot be produced by rolling.